ECOTOXICOLOGY AND ENVIRONMENTAL SAFETY 16,133-142 (1988)

The Environmental Behavior of Chemicals in Soil: Atrazine as an Example’

C. LUPI,~ A. R. BUCCHI, A. PICCIONI, AND G. A. ZAPPONI

Istituto Superiore di Sanitri, Viale Regina Elena, 299, 00161 Rome, Italy

Received January 1987

In this work a mathematical model has been developed and used to estimate the soil vertical distribution of Atrazine dispersed in the environment. Water transport, rise due to capillarity, and partition among soil-contained water, air, and organic matter, as well as degradation pro- cesses, are considered. As far as the vertical mobility in soil is concerned, the model has been derived from that proposed by P. H. Nichols, A. Walker, and R. J. Baker ((1982). Pestic. Sci. 12,484-494). Such a model has been extended to include a procedure which takes into account the Atrazine mobility due to gravitational water flow. The organic carbon (O.C.) concentration gradient in soil was also considered in the evaluation when assessing partition processes, accord- ing to models by P. J. McCall et al. (( 1983). Residue Rev. 13,231-241) and D. McKay and S. Paterson (( 1982). Environ. Sci. Technol. 16,12,654). The degradation processes are assumed to be first order, linearly related with O.C. content in soil. The application of this model to two sets of soil data demonstrated that Atrazine requires a long time (2 years or more, depending on soil features) in order to percolate at a depth comparable with those of a groundwater source. Q 1988 Academic Press, Inc.

INTRODUCTION

A mathematical model, derived from that proposed by Nichols et al. (1982) was used in order to make some tentative predictions about the time required for the contamination of groundwater following soil treatment with Atrazine.

The model of Nichols et al. was mainly directed toward interpretation of experi- mental data. The soil layer thickness studied by these authors was on the order of 20 cm. On the contrary, the efforts of the present authors were directed toward the development of a predictive model, useful to predict the effect of leaching on the contamination of groundwater sources, generally located at several meters depth in soil. The use of the model of Nichols et al. for risk assessment purposes thus required some modifications and extensions, the most important of which were as follows:

-use of calculated instead of measured values of adsorption coefficient and

-use of a compartmentalization model in order to estimate partition among air, water, and organic matter.

In order to make the model conceptually valid for application at a depth on the order of 10 m or more, the O.C. trend in soil was taken into account when calculating partition between solids and water; moreover, a procedure for the calculation of the

’ Presented at the Annual Meeting of the Society of Ecotoxicology and Environmental Safety held in Rome, Italy, 12- 14 November 1986, “Health Implications of Environmental Chemicals.”

* To whom correspondence should be addressed.

133 0147-6513/88 $3.00 Copyright 0 1988 by Academic Press, Inc. All rights of reproduction in any form reserved.

134 LUPI ET AL.

TABLE 1

MAINPROCESSESCONSIDEREDINTHESIMULATION

Process Mainly occurring Depending upon

Hydrolysis Microbial degradation

From surface to 1 - 1.5 m From surface to l-l.5 m

Reversible adsorption From surface to I- 1.5 m

Transport due to gravitational water flow

Transport due to capillarity water flow

Plant uptake

From surface to impermeable layer

Between not water- saturated layers

In the root zone of soil

pH, temperature Temperature, density of

bacterial flora Temperature O.C.

content Macroporosity,

volume, rainfall rate Field capacity, water

content Plant species,

Vegetation density

Atrazine transport by means of gravitational flow was developed. The simulation of 3 years of soil treatment with Atrazine gives useful information about time trends of distribution of Atrazine in soil, as well as information about the release of Atrazine under 4 and 8 m depth. The results demonstrated that a large amount of time is required by Atrazine in order to reach groundwater. The simulation allows some qualitative considerations about behavior of Atrazine in soil to be made.

THEORETICAL BASIS OF THE MODEL

Table 1 shows the main processes considered in the simulation. For each of these processes (excluding plant uptake) a simulation routine in BASIC language was writ- ten. Acid hydrolysis and microbial degradation were considered as a single first-order process. The model required 15 hr of computer processor time for each run of 200 days per 200 soil layers.

Adsorption and Partition between Phases

Adsorption on soil solids and partition among organic matter, water, and air con- tained in soil are the most important properties governing the physicochemical be- havior of contaminants in soil. According to data reported by Hartley and Graham- Bryce (1980) it is assumed that Atrazine partitions between solid and solution in a reversible way. These partitions can be described by the adsorption isotherm

where Qads is the quantity of solid adsorbed compound, K, is the partition coefficient, and Gate, is the quantity of substance existing in solution.

TABLE 2

ATRAZINEPHYSICOCHEMICALPARAMETERS

K, (calculated using Eq. (1)) Water solubility Half-life in soil from Nichols et al. ( 1982) KV

670 30 mm 60 days 64.600

ATRAZINE MOBILITY IN SOIL 135

MSPLACE EXCEENG WATER NT0 LOWER LAYW

FIG. 1. Gravitational flowchart. This figure schematizes the computer routine to calculate the water flow in soil due to gravitational effects and soil characteristics.

TOP LAYER

FIG. 2. Capillarity flowchart. This figure schematizes the computer routine to calculate the chemical distribution in each soil layer.

136 LUPI ET AL.

TABLE 3

METEOROL~GICALPARAMETERVALUES

Rainfall rate 10 mm/day Temperature 22°C Evaporation rate 2 mm/day

The following relationship was used in order to calculate the Atrazine K, value,

log(K,) = 3.64 - 0.55(log Ws) + 1.23 OM (Kenaga, 1980)

where Ws is water solubility (ppm) and OM is the order of magnitude. It is assumed that Atrazine behaves as an ideal solute, so partitioning between water and air accord- ing to Henry’s law is

K, = C,,JC,, = ms/ 16.04 PM = 1 /H,

where T is temperature (Kelvin), Ws is the water solubility (ppm), and PM is molecu- lar weight. Table 2 shows Atrazine partition coefficients together with physicochemi- cal parameters. These partition coefficients were adopted when using the compart- mentalization model.

Generally, organic carbon content in soil decreases very rapidly with depth (values ~0.1% under 1 m depth). This implies that Atrazine, which has a moderately high K, value, after percolation under 1- 1.5 m depth, is not significantly retained by soil solids and so becomes free to reach groundwater sources.

Chemical and Microbial Degradation in Soil

Microbial and chemical degradation has been considered a single first-order pro- cess. According to recovery values of Atrazine in soil (Nichols et al., 1982) it is as-

TABLE4

SOILPARAMETERVALUES

Soil A

O.C. content at soil surface O.C. content at 1.5 m 0.C content logarithmically Decreasing with depth Macroporosity volume Field capacity

1% 0.05%

23% 23%

Soil B

O.C. content at soil surface O.C. content at 1.5 m 0.C content logarithmically Decreasing with depth Macroporosity volume Field capacity

2% 0.1%

23% 23%

ATRAZINE MOBILITY IN SOIL 137

PPm

FIG. 3. Chemical concentration distribution in each layer after the third theoretical year of simulation, without degradation. Soil A: 10 days (A) and 200 days (B) a&r the third input. Atrazine concentration in parts per million (Y axis) and depth in meters (Xaxis).

sumed that Atrazine half-life in soil is about 60 days. As far as modeling of degrada- tion in soil is concerned, a simple linear relationship between O.C. content of soil (which decreases with depth) and Atrazine half-life is assumed.

Transport by Water Flow

Due to adsorption, transport of chemical compounds by water flow in soil may be regarded as analogous to cromatography; the compound, in fact, is at the same time transported downward and adsorbed to soil solids.

The concentration peak speed in soil for a given compound depends primarily upon water flow speed, which in turn depends upon permeability features of soil.

In general, capillary flow in soil is a very slow process, occurring mainly between soil pores having “capillary” dimensions, i.e., diameter 4 pm (Tombesi, 1968). The total volume of the capillarity porosity expressed as percentage of the total volume of soil is called field capacity. Because field capacity is a routine measurement in pedological analysis, it can be considered a useful parameter in predictive modeling.

On the contrary, gravitational water flow may be considered a rapid process. It is assumed that this flow occurs mainly between soil pores having dimensions larger than capillary, e.g., >6 pm.

In order to take into account the time required by the adsorption process, it is assumed that a complete partition among the three phases of soil occurs only within capillary flow while only a solute equilibration layer after layer is performed when simulating downward movement of water flow.

138 LUPI ET AL.

A

4-

pm

FIG. 4. Chemical concentration distribution in each layer after the third theoretical year of simulation, with degradation. Soil A: 10 days (A) and 200 days (B) after the third input. Atrazine concentration in parts per million (Y axis) and depth in meters (X axis).

A

I 1-1 I I 1 I 2 4 6 am

PPm

FIG. 5. Chemical concentration distribution in each layer after the third theoretical year of simulation, without degradation. Soil B: 10 days (A) and 200 days (B) after the third input. Atrazine concentration in parts per million (Y axis) and depth in meters (X axis).

ATBAZINE MOBILITY IN SOIL 139

A

ppm 2-

B

0

FIG. 6. Chemical concentration distribution in each layer after the third theoretical year of simulation, with degradation. Soil B: 10 days (A) and 200 days (B) after the third input. Atrazine concentration in parts per million ( Y axis) and depth in meters @axis).

In Figs. 1 and 2 the two flowcharts adopted in the model for gravitational and capillarity flow are reported.

As far as calculation of evaporation rate is concerned, the procedure suggested by Nichols et al. (1982), using calculated instead of measured values of negative pressure, has been adopted.

Plant Uptake

According to the literature (Hartley and Graham-Bryce, 1980; Walker, 1972) it is considered that the most important parameter in evaluating availability for plant

TABLE 5

ATRAZINELEACHINGUNDER~AND 8 ~DE~H:SOILA

Degradation

Total % Of last grams input

No degradation

Total 5% Of last grams input

1 st year Under 8 m 0 0.0 0 Under 4 m 0 0.0 0 ::t

2nd year Under 8 m 6 0.2 7 0.3 Under 4 m 60 2.4 94 3.8

3rd year Under 8 m 154 6.6 219 8.8 Under 4 m 367 14.6 718 28.7

140 LUPI ET AL.

TABLE 6

ATRAZINELEACHINGUNDER 4 AND 8 m DEPTH: SOIL B

Degradation No degradation

Total % Of last Total % Of last grams input grams input

-

1 st year Under 8 m 0 0.0 0 0.0 Under 4 m 0 0.0 0 0.0

2nd year Under 8 m 0 0.0 0 0.0 Under 4 m 1 0.1 3 0.2

3rd year Under 8 m 10 0.3 23 I.0 Under 4 m 22 0.6 62 2.5

uptake is the concentration in soil water. The depth at which plant uptake is more effective depends upon plant species. For a grass-covered soil, the root zone depth was assumed to be about 100 cm.

Model Parameter

In Table 3 meteorological parameter used in the simulation is reported. Rainfall rate, temperature, and evaporation rate were assumed to be constant.

Each simulation consisted of three theoretical years, each 200 days long. On the first day of each theoretical year, a soil treatment with 2.5 kg/ha of Atrazine, evenly distributed in the first layer of soil, was simulated. Two sets of soil data (Table 4) were used in the simulation, each in the absence or presence of degradation.

In order to evaluate the influence of O.C. content in soil on the transport processes, the only variable between the two soil types was the O.C. content of soil. Each soil was divided in 200 layers, each 4 cm thick.

RESULTS

In Figs. 3,4,5, and 6 the trends of concentration in soil water for the third theoreti- cal year of simulation are reported. Tables 5 and 6 show the rate of Atrazine leaching under 4 and 8 m depth for soil A and soil B.

Except in the case of soil A, in the absence of degradation processes, a very small quantity of Atrazine is able to be transported to any depth. More specifically, Atrazine required at least 2 years in order to percolate significantly under 4 m in the lowest O.C. content soil, and at least 3 years in order to percolate significantly under 4 m in soil with the highest O.C. content. The examination of the concentration trend in soil water demonstrates that when a yearly treatment of a soil surface with Atrazine is performed, the concentration peak is never allowed to move under the first meter of depth in the soil, thus always remaining available for plants.

DISCUSSION

Despite the fact that preliminary comparison with experimental data (Basile and Scognamiglio, 1983) indicated agreement with results provided by the model (Fig.

ATRAZII’E MOBILITY IN SOIL 141

wm 3.0 -

\ 2.5 - ’ \

\ 2.0- ’

\ \

15

Pm 1

80 120 days

0

15 60 120

days

cm cm

cm

FIG. 7. Comparison between experimental data (Basile and Scognamiglio, 1983), A, and theoretical data as computed with the model, B.

7), in the absence of a more systematic validation of the model, the quantitative data provided should be examined very cautiously.

The following qualitative results are very interesting from a risk assessment point of view.

1. Atrazine percolation in soil is a very slow process. Despite the fact that the values of permeability and rainfall rate used in the model are likely to overestimate the real values, Atrazine will percolate under a depth comparable with that of a groundwater source 2 years after the first treatment of soil.

142 LUPI ET AL.

2. This also implies that the decontamination process would require a similar amount of time.

3. Continued treatment of soil with Atrazine, repeated for several years, may result in the permanent contamination of a soil surface (as some monocoltures require) and the time-delayed contamination of a groundwater source.

4. Soil O.C. is very effective in reducing leaching of Atrazine in soil. For a given amount of water flow, differing soil O.C. contents may result in very different mobil- ity features of Atrazine.

REFERENCES

BASILE, G., AND SCOGNAMIGLIO, D. (1983). Mobilita e degradazione di residui di Atrazina nel suolo. Inquinamento 7/t&39-41.

HARTLEY, G. S., AND GRAHAM-BRYCE, I. J. (1980). In Physical Principles in Pesticide Behaviour. Aca- demic Press, London/San Diego.

KENAGA. E. E. (1980). Predicted bioconcentration factors and soil sorption coefficient of pesticides and other chemicals. Ecotoxicol. Environ. Safety 4,26-38.

MCCALL, P. J., et al. (1983). Estimation of environmental partitioning of organic chemicals in model ecosystems. Residue Rev. 13,23 l-24 1.

MCKAY, D., AND PATERSON, S. (1982). Fugacity revisited. Environ. Sci. Technol. 16, 12,654. NICHOLS, P. H., WALKER, A., AND BAKER, R. J. (1982). Measurement and simulation of the movement

and degradation of atrazine and metribuzin in a fallow soil. Pestic. Sci. 12,484-494. TOMBESI, L. (1968). In Elementi di scienza de1 suolo. Edagricole, Bologna. WALKER, A. (1972). Availability of atrazine to plants in different soils. Pestic. Sci. 3, 139.